At radio frequencies, from low frequencies into the GHz range, vector network analyzers (VNA) are used for the accurate measurement of active and passive circuits. Among other uses, VNAs are used in antenna ranges in order to characterize antenna devices.
A VNA typically measures the so-called scattering parameters of n-port devices (n=1, 2, . . .), which are optionally converted into other sets of multiport parameters (for example, Z-parameters or Y-parameters). However, at high frequencies this measured data may have substantial measurement errors. Error correction procedure for the VNA were developed, which ensure that accurate measurements of high-frequency electronic components are realizable. The measurement accuracy of VNAs depends primarily upon the availability of a method for system-error correction. The correction process is also called in the art “calibration” or “deembedding.”
In the case of error correction, within the so-called calibration process, devices under test (DUT), which are known in part or in their entirety, are measured with regard to reflection and/or transmission behavior. Correction data (so-called error-values or coefficients) are obtained from these measured values, via special computational methods. With these correction data and a corresponding correction calculation, corrected measured values are obtained for every required device under test.
The conventional form of description for the electrical behavior of components and circuits in high-frequency technology is provided via the scattering parameters (also referred to as S-parameters). The scattering parameters relate wave quantities rather than currents and voltages. This presentation is particularly well adapted to the physical circumstances of high-frequency technology. If required, these scattering parameters can be converted into other electrical network parameters, which link currents and voltages.
Antennas are usually characterized in terms of antenna gain (transmission) and of reflection from the antenna feed port only. This characterization disregards the reflection of the incoming radiation from the antenna. These reflections are substantial in measurement systems operating in the near field of the antenna and therefore there is an interest in characterizing the antennas as a full 2-port device.
As shown in FIG. 1, an RF antennas' array system 100, as known in the art comprises three parts: a Radio Frequency (RF) Signals measurement unit such as a VNA 110 configured to generate and transmit a number of RF signals and measure the received/reflected signals; an array unit 120 comprising one or more antennas 125 which transfer the RF signals to propagating a wave in the transmit-path and back in the receive-path; and routing electronic RF components such as cables, connectors, splitters, attenuators and switches which are used to connect the network analyzer ports 115 to the antenna array unit 120. The exemplary antenna array system 100 can be a MIMO radar system which characterizes the properties of a DUT 140 in order to assess its shape or composition from the electromagnetic wave reflections.
In network-analyzer based systems, it is common to model the system using ‘signal flow graph’ which describes the system's transmitted and reflected signals at the input and output of each component in the system and the component may be modeled using set of transmission and reflection coefficients. Measurement errors are typically caused by the system's routing electronic elements (such as the switches, ports, couplers etc.). In order to compensate the measurement errors a calibration process is required.
The prior art solutions that are currently used, include specifically a calibration flow for RF array measures which corrects only the routing electronic of the array without the antennas.